Is contrast needed for Magnetic Resonance Imaging (MRI) to evaluate a foreign body in the foot?

From the Guidelines

Contrast is not typically needed for an MRI to detect a foreign body in the foot, as a standard non-contrast MRI is usually sufficient to identify most foreign bodies, especially those that are non-metallic such as wood, plastic, or glass. The high soft tissue resolution of MRI allows for excellent visualization of surrounding inflammation, edema, and any associated complications like abscess formation, which can help locate even small foreign bodies 1. Metal objects are actually better visualized on plain radiographs (X-rays) or CT scans due to their distinctive appearance and the artifact they create on MRI 1.

Key Considerations

• In certain complex cases where there is significant inflammation or when the foreign body has been present for a long time, contrast may occasionally be added to better delineate abscesses or to distinguish between active and chronic inflammation 1.
• However, this is determined on a case-by-case basis by the radiologist.
• If metallic foreign bodies are suspected, it's essential to rule them out with X-rays before proceeding with MRI to avoid potential movement of the object during the scan or heating of the metal, which could cause additional tissue damage 1.
• Ultrasound (US) is also a useful imaging tool to exclude a foreign body in the setting of penetrating trauma to the foot, especially if the foreign body is not radiopaque (e.g., wood or plastic), with a reported 90% sensitivity for visualizing wooden foreign bodies in some clinical and experimental studies 1.

Imaging Modalities

• CT scans have high sensitivity for detection of radiopaque foreign bodies and are superior to MRI in identifying water-rich fresh wood 1.
• MRI has lower sensitivity compared to CT for detection of foreign bodies but can be useful in certain circumstances, such as with complicated foreign bodies or to exclude associated osteomyelitis or abscess 1.
• US allows for better localization of radiopaque foreign bodies, assessment of tendons and vascular structures, and US-guided removal of the foreign body 1.

From the Research

Evaluation of Foreign Bodies in the Foot using MRI

• The use of contrast in Magnetic Resonance Imaging (MRI) for evaluating foreign bodies in the foot is not explicitly mentioned in the provided studies as a necessary component 2, 3.
• A study from 1993 suggests that MRI is probably equal to computed tomography (CT) in visualizing and localizing foreign objects in the foot, but it is less readily available and more expensive, and contraindicated with ferrous metal foreign bodies 2.
• Another study from 2013 found that the sensitivity of MRI for foreign body detection was 58%, and it was highly specific, but the detection rate depended on the type of foreign body and the susceptibility artefact 3.
• To identify foreign bodies with MRI, pulse sequences should be used to enhance the susceptibility artefact, but the study does not mention the need for contrast 3.
• Other imaging modalities, such as ultrasound and CT, may be more effective or practical for evaluating foreign bodies in the foot, depending on the type of foreign body and its location 4, 2, 3.